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Hierarchical Self-Assembly of a Porphyrin into Chiral Macroscopic Flowers with Superhydrophobic and Enantioselective Property Hejin Jiang,†,∥ Li Zhang,*,† Jie Chen,† and Minghua Liu*,†,‡,§,∥ †
Beijing National Laboratory for Molecular Science (BNLMS), CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ National Center for Nanoscience and Technology, Beijing 100190, China § Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China ∥ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: Supramolecular self-assembly provides an efficient way to fabricate simple units into various hierarchical nano/microstructures, which could mimic the bioself-assembly and develop functional materials. Since chiral molecules and chiral nanostructures are widely adopted by biological systems, an introduction of the chiral factor into the self-assembly process will provide better understanding of the biological systems. Here, using a chiral amphiphilic histidine to assist the self-assembly of a porphyrin with four carboxylic acids, we obtained hierarchical chiral nano- to microstructures. We have found that through the hydrogen bonds/electrostatic interactions between the porphyrin and histidine derivatives, the π−π stacking between the porphyrins, and hydrophobic interactions between the amphiphilic histidine, the two components could self-assemble into chiral nanohelices and microflowers. The supramolecular chirality of these structures was confirmed by scanning electron microscopy images as well as the circular dichroism spectra, which was found to follow the molecular chirality of the histidine derivative. More interestingly, the microflower structures formed a superhydrophobic and chiral surface, which exhibited macroscopic enantioselective recognition of some L- and D-amino acids via contact angle measurements. KEYWORDS: self-assembly, porphyrin, chiral, microflowers, enantioselectivity
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understanding between molecular and supramolecular as well as nanoscale chirality.23,24 However, it still remains an important issue if the controllability of the molecular chirality can be extended to even a larger scale and how chirality is transferred during such process. Some recent works show such possibility.15,25 For example, McKee and co-workers obtained the chiral, macroscopic vaterite toroidal suprastructure of calcium carbonate, which could show counterclockwise or clockwise spiraling morphology via the inductions of different enantiomeric amino acids.25 Kotov et al. have assembled mesoscale helices with near-unity enantiomeric excess using
hirality, as a ubiquitous phenomenon in nature, is expressed in biological systems at various hierarchical levels like molecular amino acid, sugar, supramolecular DNA, protein, and microtubular and cell structures.1,2 These structures are strongly related to their functions such as the duplicate of the DNA and the enzymatic reaction of the proteins. Self-assembly provides an efficient way to obtain hierarchical nano- to microstructures starting from simple molecular units.3,4 Since most of biological systems exhibit chiral sense, the chiral self-assembly based on or assisted by the chiral components could provide better understanding and mimic the biological self-assembly.5,6 For instance, some peptides,7−9 amino acid derivatives,10−12 sugar derivatives,13 nanoparticles,14−16 and even some achiral molecules17,18 have been designed and self-assembled into various chiral nanostructures19 or hybrid materials.20−22 These provide the well © 2017 American Chemical Society
Received: September 12, 2017 Accepted: November 22, 2017 Published: November 22, 2017 12453
DOI: 10.1021/acsnano.7b06484 ACS Nano 2017, 11, 12453−12460
Article
Cite This: ACS Nano 2017, 11, 12453−12460
Article
ACS Nano chiral semiconductors.15 On the other hand, flower-shaped structures generally possess large surface area, easy accessibility to reaction sites, confinement effect, and hierarchical nanostructures extending to a larger scale, which could possibly exhibit enhanced enzyme activity26 and superhydrophobicity.27 Although some flower-shaped structures have been obtained in self-assemblies,28 chiral flower-like structures have rarely been reported in pure organic systems. Herein, chiral flower-shaped structures were fabricated based on the self-assembly of porphyrins, and their surface properties were investigated. Porphyrin is a well-known building block that has heterocyclic macrocycle ring and a large π-conjugated system, which has been widely used in the catalysis, photovoltaic devices, and chemical sensors.29,30 Their planar aromatic macrocycle is optimal for π−π stacking, and it is a versatile platform for peripheral decoration with groups that can offer other interaction motifs, such as metal−ligand coordination, hydrogen bonding, and electrostatic interaction. By virtue of these noncovalent interactions, porphyrin is extensively used as a brick to fabricate various structures such as hollow hexagonal nanoprisms, nanodiscs, nanorods, nanowires, nanospheres, vesicles, nanoarrays, and showed some special or enhanced functions.31−45 Meanwhile, porphyrin plays an important role in organisms such as hemoglobin, vitamin B12, cytochrome, and chlorophylls in human, plants, and microorganism. Because of their significance in many biological processes, it is expected to assemble the porphyrin in a chiral environment or into chiral nanostructures. So far, only rare examples of chiral porphyrin structures have been reported, in which porphyrin derivatives are directly linked to the chiral units by covalent bonds.46−49 Here, by means of supramolecular interactions, we construct the chiral self-assembly of an achiral porphyrin assisted by an chiral amphiphilic histidine. The amphiphilic histidine, abbreviated as LHC18 or DHC18, has a chiral center and easily interacts with carboxylic acid groups through the interaction between the imidazole and carboxylic acid groups.50 Herein, tetrakis(4-carboxyphenyl)porphyrin (TCPP) is chosen to co-assemble with LHC18 and DHC18. By adjusting the molar ratio of the two components and the mixed solvents, we obtained chiral nano/microstructures, as verified via the scanning electron microscopy (SEM) and further by circular dichroism (CD) spectra. Interestingly, the chiral structures can be extended to the microscale level, and the handedness of the structures was controlled by the molecular chirality of the amphiphiles, as shown in Figure 1. Moreover, the chiral flower showed a superhydrophobic surface due to the hierarchical nano/ microstructures. Interestingly, the chiral microflowers further exhibited enantioselectivity to aspartic acids, one of the proteinogenic amino acids. So far, although various porphyrin nano- and microstructures have been reported, the chiral architectures and corresponding functions were rarely explored. This paper provids an example of chiral microflower with superhydrophobic and enantioselective property.
Figure 1. Molecular structures of amphiphilic histidine derivative (LHC18 or DHC18) and TCPP and illustration on self-assembly of amphiphilic histidine/TCPP into chiral microflowers, in which the chiral sense originates from preference stacking of nanosheets and is determined by molecular chirality of histidine derivatives.
aggregates by the aid of histidine derivatives. Figure 2 shows the nanostructures formed by the self-assembly of LHC18/TCPP under various molar ratios of LHC18 to TCPP. LHC18 could self-assemble into entangled nanobelt structures with the width of about 300 nm in DMF/H2O mixed solvent, as shown in Figure S1a. On the other hand, individual TCPP could form straight nanofibers in DMF/H2O solution (Figure S1b). Upon mixing TCPP with LHC18 or DHC18, significant changes in the morphologies are observed due to the interactions between the carboxylic acid group of TCPP and the histidine moiety of LHC18. The formed nanostructures can be regulated by the molar ratio of LHC18 to TCPP. Non-uniform amorphous structures can be obtained under the ratio of LHC18 to TCPP at 1:1. By increasing of molar ratio to 2:1, obvious chiral twists with right-handed sense are observed. The pitch of twist is about 470 nm, and the width is around 200 nm. When the molar ratio reaches to 3:1, bundles of nanotwists formed by single nanotwist entangled together are observed. It is interesting to note that the flower-like structures appear when the molar ratio increasing to 4:1. These flower-like structures consisted of bent or curved nanolayers. This result demonstrates that molar ratio of the two components has a great effect on the morphology of co-assemblies. It should be noted that the LHC18-TCPP heteroaggregates can be formed when stoichiometry of the LHC18/TCPP is at 2:1, 3:1, and 4:1, according to a Job’s plot analysis (Figure S2). However, the flower-like structures were only observed at a ratio of 4:1, so we selected the ratio of 4:1 as an optimal condition to fabricate the flower structures. When DHC18 was used to co-assemble with TCPP, similar nano/microstructures were obtained. However, their handedness of the chiral nanostructures was just in a mirror image with that of LHC18/TCPP assemblies. This suggests that the supramolecular chirality of assemblies follows molecular chirality of amphiphilic histidine (Figure S3). The effect of solvent on the co-assemblies is also investigated. Using LHC18 to TCPP at 4:1 as an example, the co-assemblies formed three kinds of structures with the variation of volume ratio of the miscible solvents. As shown in Figure 2, when the ratio of the solvents (DMF/H2O) is
